1. Technical Field
This disclosure relates to detectors of voltage peaks of oscillating signals and more particularly to a novel architecture, realizable in a completely integrated form, adapted to generate an envelope voltage corresponding to the instantaneous peak value of an input oscillating voltage and to a related method.
2. Description of the Related Art
Forced switching power supplies and, more particularly, systems for active power factor correction (PFC), store information about peak values of an input voltage, that typically is the voltage of the mains, thus at a low frequency.
In general, PFC pre-regulators are switching converters controlled such to obtain a regulated DC output voltage from an input AC voltage. Using particular switching techniques, PFC regulators are capable of absorbing a sinusoidal current in phase with the voltage of the mains, thus obtaining in this way a power factor close to 1 and a reduced total harmonic distortion of the current absorbed from the mains.
FIG. 1 is an example of a known PFC pre-regulator with a “Transition Mode” control.
The amplifier VA compares a fraction of the output voltage with an internal reference voltage VREF for generating an error signal that is sent to the multiplier.
The multiplier MULTIPLIER carries out the product between a fraction of the mains voltage and the output signal of the amplifier VA, thus outputting a sinusoidal signal in phase with the mains voltage and having an amplitude proportional to the error signal itself.
The PWM comparator compares the signal generated by the multiplier with a value proportional to the current flowing through the inductor L and turns off the power MOSFET M as soon as the two values match each other, thus determining the envelope of the current through the inductor itself.
Once the MOSFET M is off, the inductor L discharges through the load the energy stored during the previous phase. At this point, the MOSFET M is turned on again by the switching of the zero-cross comparator ZCD and the loop restarts.
The current absorbed from the mains, because of the input filter, will be the low-pass component of the current flowing throughout the inductor L, thus its mean value at each switching cycle, equal to one half of the envelope of the peaks and with a sinusoidal waveform in phase with the mains voltage itself, as shown in FIG. 2.
From an analysis of the functioning, it is evident that the gain of the power stage of a PFC pre-regulator depends with a quadratic law from the RMS value of the mains voltage. In case of fluctuations of the mains voltage, the error amplifier intervenes in an appropriate manner for bringing the sinusoidal reference (input to the PWM comparator) to the value that obtains a correct regulation of the output.
This quadratic function that ties the gain to the value of the input voltage causes the followings drawbacks:                the error amplifier has linear dynamics in a very extended range. In systems with a so-called universal supply the input voltage may vary by a factor 3 or more, thus the gain may vary by a factor 9. Therefore the error amplifier, for a same load, should be capable of reducing its output at least by nine times;        the quadratic variation of the gain implies a similar variation of the cut-off frequency of the open loop transfer function, with consequent difficulty of compensating the system and a relatively slow dynamical response when functioning at the maximum voltage. Indeed, the frequency response of the system has a single pole. This pole is independent from the input voltage and is tied to the resistance and to the capacitance on the output of the pre-regulator. Therefore, if the error amplifier is compensated for having a band of 20 Hz for the open loop transfer function at the maximum voltage, the band will be of about 2 Hz at the minimum mains voltage, thus causing an even slower dynamical response;        undershoots/overshoots of the output voltage of the pre-regulator, in response to great fluctuations of the mains voltage. With the same load, at each variation of the input voltage, in order to make the system remain regulated, there should be a corresponding opposite variation of the output of the error amplifier. The amplifier is relatively slow thus, before being capable of following and compensating the variation, output undershoots/overshoots may occur.        
In order to compensate these phenomena, a compensation factor can be introduced, in the loop gain, which is inversely proportional to the square of the input voltage. This compensation technique, called “voltage feedforward”, consists in deriving a voltage proportional to the RMS value of the input voltage, providing this value to a squaring/dividing circuit (corrector 1/VFF2) and providing the resulting signal to the multiplier that generates the reference for the peak current of the system.
With this technique, a variation of the supply voltage causes a variation inversely proportional to the amplitude of the sinusoid generated by the multiplier; if the supply voltage doubles, the amplitude of the signal generated by the multiplier halves and vice versa. The reference for the peak current is, in this way, immediately adapted to the new working conditions without need of intervention of the error amplifier. The loop gain will remain constant for any value of the input voltage, thus sensibly improving the dynamical behavior of the pre-regulator. Moreover, the design of the external network for ensuring the stability of the system is simplified.
From the above considerations, the circuit for sensing the RMS value (peak detector) is fully effective if it is capable of following fluctuations of the input voltage in both directions. A fast detection of peaks may be insufficient when they increase but also when their value decreases. Indeed, if the detection of the peak reduction of the mains voltage is very slow, the setting of the correct feedforward action will be delayed, with a consequent excessive overshoot of the output voltage of the pre-regulator because of great variations of the supply voltage.
Commonly, as disclosed in U.S. Pat. No. 7,239,120, and employed in controller L6563 of STMicroelectronics, in order to obtain this function, a so-called integrated “ideal diode” is used, comprising an operational amplifier configured as voltage follower in the feedback path, with an external capacitor CFF for storing information and an external resistance RFF as shown in FIG. 3.
The resistance RFF, properly determined, provides the discharge path of the capacitor and makes the system capable of adapting itself, with a time constant RFFCFF, to reductions of the root mean square value of the input voltage. The time constant RFFCFF is determined such to make the discharge phenomenon not detectable inside each half period of the mains voltage; the RMS value of the mains voltage is thus close to a continuous value.
A drawback of this type of circuit, besides using two discrete external components, consists in that the system responds according to an exponential law with a time constant RFFCFF that, for the reasons stated above, will be relatively great (typically in the order of several hundreds of ms). This implies a loss of effectiveness of the feedforward technique for a longer time the greater the variation of the input voltage and thus the greater the time constant RFFCFF.
A mains drop detector, shown in FIG. 4, used in the integrated control L6564 of STMicroelectronics, stores on an inner capacitance C1 the peak of a scaled replica of the mains voltage (excluding any voltage offset).
The voltage on this capacitance, called VFFi, is used as threshold of a comparator that compares it with a peak voltage VFF (minus a voltage drop across a resistor R1. The threshold and the external RC filter RFFCFF are dimensioned such that, in a mains voltage period, the voltage VFF does not decrease sufficiently to switch the comparator. Should an abrupt decrease of the mains voltage occur, the voltage on the external capacitor CFF, after a certain number of periods, drops below the threshold thus switching the comparator that, on its turn, turns on transistor M6 that acts as a fast discharge circuit of the capacitance CFF, which will be charged with a new peak value.